Browsing by Author "Lenzer, T."

New experimental results for the collisional energy transfer of highly vibrationally excited toluene and pyrazine employing the method of "kinetically controlled selective ionization (KCSI)" are presented. By means of a master equation approach we determine complete and detailed collisional transition probabilities P(E',E) for energies up to 50 000 cm(-1). The same monoexponential representation P(E',E) proportional to exp[ - ((E - E')/alpha (1)(E))(Y)] (for E' less than or equal to E) with a parametric exponent Y in the argument and linearly energy dependent alpha (1)(E) = C-0 + C1E successfully used in our earlier investigation [T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, J. Chem. Phys., 2000, 112, 4090] can reproduce the toluene and pyrazine results for the whole range of bath gases studied. The parameters Y, C-0 and C-1 of P(E',E) show a smooth increase with the size of the collider. An approximately linear energy dependence of the first moment of energy transfer [DeltaE] is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly smaller - [DeltaE] values outside the uncertainty limits of the KCSI results. It is shown that this can mainly be traced back to the critical dependence of the IRF data on small uncertainties in the calibration curve. Some of the trends with respect to the energy transfer efficiencies of different colliders observed in the KCSI experiments are easily rationalized on the basis of accompanying trajectory calculations on the deactivation of highly vibrationally excited pyrazine by n-propane and CO2. The negligible influence of the V-V relaxation channel in the pyrazine + CO2 system observed in earlier IR diode laser studies is confirmed.

Complete experimental transition probability density functions P(E-',E) have been determined for collisions between highly vibrationally excited azulene and several bath gases over a wide energy range. This was achieved by applying 2-color "kinetically controlled selective ionization (KCSI)" [U. Hold, T. Lenzer, K. Luther, K. Reihs, and A. C. Symonds, J. Chem. Phys. 112, 4076 (2000)]. The results are "self-calibrating," i.e., independent of any empirical calibration curve, as usually needed in traditional energy transfer experiments like time-resolved ultraviolet absorption or infrared fluorescence. The complete data set can be described by our recently introduced monoexponential 3-parameter P(E-',E) form with a parametric exponent Y in the argument, P(E-',E)proportional toexp[-{(E-E-')/(C-0+C-1.E)}(Y)]. For small colliders (helium, argon, xenon, N-2, and CO2) the P(E-',E) show increased amplitudes in the wings compared to a monoexponential form (Y<1). For larger colliders, the wings of P(E-',E) have an even smaller amplitude (Y>1) than that provided by a monoexponential. Approximate simulations show that the wings of P(E-',E) at amplitudes <1x10(-6) (cm(-1))(-1) have a nearly negligible influence on the population distributions and the net energy transfer. All optimized P(E-',E) representations exhibit a linear energy dependence of the collision parameter alpha(1)(E)=C-0+C-1.E, which also results in an (approximately) linear dependence of and (1/2). The energy transfer parameters presented in this study have benchmark character in certainty and accuracy, e.g., with only 2%-5% uncertainty for our data below 25 000 cm(-1). Deviations of previously reported first moment data from ultraviolet absorption and infrared fluorescence measurements can be traced back to either the influence of azulene self-collisions or well-known uncertainties in calibration curves. (C) 2003 American Institute of Physics.

The method of kinetically controlled selective ionization (KCSI) for investigating collisional energy transfer in highly vibrationally excited molecules is presented in detail. In this first paper of a series the focus is on the key concepts and the technical realization of KCSI experiments to provide a common basis for following reports on our available results of KCSI studies on the vibrational relaxation of a variety of larger molecules. The KCSI technique directly monitors the energetic position and shape of the population distributions g(E,t) during the relaxation process by means of an energy selective two photon ionization process via an electronic intermediate state. Such measurements allow-for the first time-to extract complete and accurate experimental sets of transition probability distributions P(E',E) even at quasicontinuous densities of states. Basic energy transfer quantities are already obtained from a straightforward analysis of the arrival time and width of the KCSI curves. A master equation formalism is outlined which is the basis of a data inversion providing a complete evaluation of the experimental information content. Various examples of characteristic KCSI data on collisional deactivation of highly vibrationally excited molecular populations are used to discuss important aspects of the quality and the general character of P(E-',E) parameters deduced from such measurements. The conditions for a successful modeling of these data are very tightly bound, and the resulting energy transfer parameters are therefore of high precision. In Paper II [J. Chem. Phys. 112, 4090 (2000), following article] we give a full account of the toluene KCSI experiments. We will deal with our completed studies on azulene, azulene-d(8), pyrazine and pyridine in follow-up publications of this series. (C) 2000 American Institute of Physics. [S0021-9606(00)01504-X].

Complete and detailed experimental transition probability density functions P(E',E) have been determined for the first time for collisions between a large, highly vibrationally excited molecule, toluene, and several bath gases. This was achieved by applying the method of kinetically controlled selective ionization (KCSI) (Paper I [J. Chem. Phys. 112, 4076 (2000), preceding article]). An optimum P(E-',E) representation is recommended (monoexponential with a parametric exponent in the argument) which uses only three parameters and features a smooth behavior of all parameters for the entire set of bath gases. In helium, argon, and CO2 the P(E-',E) show relatively increased amplitudes in the wings-large energy gaps \E'-E\-which can also be represented by a biexponential form. The fractional contribution of the second exponent in these biexponentials, which is directly related to the fraction of the so-called "supercollisions," is found to be very small (< 0.1%). For larger colliders the second term disappears completely and the wings of P(E-',E) have an even smaller amplitude than that provided by a monoexponential form. At such low levels, the second exponent is therefore of practically no relevance for the overall energy relaxation rate. All optimized P(E-',E) representations show a marked linear energetic dependence of the (weak) collision parameter alpha(1)(E), which also results in an (approximately) linear dependence of and of the square root of . The energy transfer parameters presented in this study form a new benchmark class in certainty and accuracy, e.g., with only 2%-7% uncertainty for our data below 25 000 cm(-1). They should also form a reliable testground for future trajectory calculations and theories describing collisional energy transfer of polyatomic molecules. (C) 2000 American Institute of Physics. [S0021-9606(00)01604-4].

Transient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1-3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range 330-390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals by collisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally "hot" and "cold" CH2I and CH2I2 were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH2I radicals between 330 and 400 nm was reconstructed from the simulated concentration-time profiles. The evolution of the absorption spectra of CH2I radicals and CH2I2 due to collisional energy transfer was simulated in the framework of a modified Sulzer-Wieland model. Additional master equation simulations for the collisional deactivation of CH2I by CH2I2 yield values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH2I radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2 in dense fluids are discussed.

Direct measurements of the gas-phase collisional energy transfer parameters are reported for the deactivation of highly vibrationally excited trans-stilbene molecules, initially prepared with an average energy of about 40 000 cm(-1), in the bath gases argon, CO2, and n-heptane. The method of kinetically controlled selective ionization (KCSI) has been used. Complete experimental collisional transition probability density functions P(E',E) are determined, which are represented by a monoexponential form with a parametric exponent in the argument, P(E',E) proportional to exp[-{(E - E')/(C-0 + C1E)}(Y)] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, k(E,1), or of resulting first moments of energy transfer, (Delta E(E)), for trans-stilbene with those for azulene and toluene clearly shows the considerably more efficient deactivation of trans-stilbene for all bath gases, presumably due to the much greater number of very low-frequency modes of trans-stilbene. However, on a relative scale this gain in deactivation rate of excited trans-stilbene is clearly collider dependent and decreases distinctly with the growing collision efficiency of the larger bath gas molecules.

A new detection method for obtaining collisional transition probabilities P (E', E) of highly vibrationally excited molecules in the gas phase is presented. The technique employs energy-selective probing of the time-dependent vibrational population distribution by kinetically controlled selective fluorescence (KCSF)". We present experimental results for a test system, the collisional deactivation of toluene by argon, where we use the well-known kinetically controlled selective ionization (KCSI) scheme as a reference for comparison. A newly designed setup is employed that allows simultaneous detection of fluorescence and ionization signals under identical experimental conditions ( kinetically controlled selective fluorescence and ionization = KCSFI"). For the system toluene + argon it is demonstrated that KCSF and KCSI yield identical results. A rate-equation model is presented to understand common features and differences of both approaches. The fluorescence detection scheme shows promise for future investigations on collisional energy transfer. The experimental setup is simpler, because it requires no additional ionization wavelength. This will hopefully give access to the P ( E 0, E) of systems where, e. g., ionization schemes are difficult to implement due to short wavelengths required for the ionization step. A few examples will be outlined briefly.

It is shown that the spread among the various "direct'' experimental data in the literature, so unsatisfactory for their application in chemical kinetics, can be removed consistently. Underlying agreement within very small uncertainties is demonstrated for the case of the much studied collisional relaxation of highly vibrationally excited azulene. Benchmark experimental data for the collisional energy transfer of highly vibrationally excited azulene obtained by the method of "kinetically controlled selective ionization (KCSI)'' (U. Hold, T. Lenzer, K. Luther and A. C. Symonds, J. Chem. Phys., 2003, 119, 11192) are used for a detailed comparison with earlier measurements employing time-resolved ultraviolet absorption (UVA) and infrared fluorescence (IRF). The experimental UVA and IRF traces are simulated by convolution of the transient vibrational distributions g(E) during relaxation obtained from KCSI measurements with the respective calibration curves of the UVA and IRF experiments. The differences between such simulations and the experimental curves are traced back to non-negligible contributions of azulene self-collisions in the UVA and IRF data. Astonishing quantitative agreement is reached when azulene/bath gas mixing ratios of the corresponding UVA/IRF experiments are fully accounted for in the KCSI simulations. The influence of self-collisions is thus quantitatively assessed as an important source of error in addition to the well-known problem of calibration curve uncertainties in UVA and IRF detection as discussed earlier (T. Lenzer, K. Luther, K. Reihs and A.C. Symonds, J. Chem. Phys., 2000, 112, 4090).

The formation of particles by the rapid expansion of supercritical solutions (RESS) of four organic substances (n-undecane, naphthalene, trans-stilbene and benzoic acid) dissolved in supercritical CO2 has been investigated employing two complementary on-line in situ detection techniques. For the first time, a laser-based shadowgraphy (LABS) setup was applied, which allows the recording of the diameter, morphology, size distribution and concentration of particles down to sizes of roughly 8 mum. In addition, laser-based three-wavelength extinction measurements (3-WEM) have been employed to determine the average particle diameter, the width of the particle size distribution and the concentration. The average particle sizes determined from 3-WEM for n-undecane, naphthalene, trans-stilbene and benzoic acid are 2660, 1260, 550 and 430 nm, respectively, and the standard deviation of the (assumed) logarithmic normal distributions 0.37, 0.42, 0.78 and 0.60. The particle size distributions at large diameters from LABS compare favourably with the tails of the 3-WEM distributions. An analysis of the morphology of trans-stilbene particles and n-undecane droplets reveals an, on average, slightly elliptical shape.

Experimental collisional energy transfer data from kinetically controlled selective ionization (KCSI) and ultraviolet absorption (UVA) experiments are analyzed in the framework of the partially ergodic collision theory (PECT). Collisions of azulene and biphenylene with different colliders are investigated as case studies. The downward wings of the P(E',E) energy transfer distributions obtained from the PECT model are fitted to the recently introduced "variable-shape"-exponential 3-parameter functional form of P(E,E) obtained from KCSI experiments, P(E',E) &PROP; exp[-{(E - E')/(C-0 + C1E)}(Y)]. The PECT model is able to reproduce the characteristic dependence of the KCSI "shape parameter" Y on the choice of collider, the energy dependent width of the KCSI P(E',E) distributions, described by (α(E) = C-0 + C1E, and the temperature dependence of the UVA data above room temperature. The statistical approach of PECT obviously captures the essence of large molecule energy transfer at chemically significant energies without the need of knowing specific features of the detailed collision dynamics. It therefore shows promise for predicting the shape of P(E,E) in master equation kernels for larger molecules.

The temperature dependence of the gas-phase collisional relaxation of highly vibrationally excited aromatic molecules has been studied using large scale classical trajectory calculations. The investigations have focused on azulene collisions with different colliders (He, Ar and N-2) as well as pyrazine self-collisions providing the moments of energy transfer (Delta E) and (Delta E-2) in the temperature range 50-1500 K. The interaction well depth epsilon(eff)/k(B) is found to be the key factor controlling the observed T dependence of collisional energy transfer. Systems with a relatively deep interaction well (pyrazine + pyrazine, azulene + Ar, azulene + N-2) show a pronounced negative dependence of - (Delta E) when T < epsilon(eff)/k(B) (in the systems studied here roughly at T < 300-400 K). The increased efficiency of collisional energy transfer at low T is due to additional contributions from collisions at large impact parameters. In systems with a very shallow well (azulene + He), however, a positive T dependence is found in the low temperature regime (<300 K) due to the dominant contributions from impulsive, adiabatic collisions at short impact parameters. At higher temperatures (T > 300-400 K) - when the temperature is well above epsilon(eff)/k(B) - all systems behave qualitatively similar, showing only a very weak, slightly negative T dependence, as long as one is still far away from thermal equilibrium.

The ultrafast internal conversion (IQ dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S-2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S-2 lifetime tau(2) on the order of 100-200 fs are estimated. The S-1 lifetime tau(1) varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments Of C-40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau(1) for citranaxanthin are discussed.

The ultrafast internal conversion ( IC) dynamics of seven C-40 carotenoids have been investigated at room temperature in a variety of solvents using two- color transient lens ( TL) pump - probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and - for the. first time - echinenone, as well as new data for lycopene, beta- carotene, ( 3R, 3'R)- zeaxanthin and ( 3R, 3'R, 6'R)- lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S-2 band at 400 nm. For the S-2 lifetime solvent- independent upper limits on the order of 100 - 200 fs are estimated for all carotenoids studied. The S-1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/ echinenone/ b- carotene ( 13/ 12/ 11 double bonds) one. finds tau(1) approximate to 5, 7.7 and 9 ps for the S-1 -> S-0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau(1), as observed for canthaxanthin/ astaxanthin ( tau(1) approximate to 5 ps in both cases). For all carotenoids studied, tau(1) is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e. g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C-40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau(1).